NR AMMV
AU Wilesmith,J.W.
TI Creutzfeldt-Jakob disease and bovine spongiform encephalopathy. Cohort study of cows is in progress.
QU British Medical Journal 1996 Mar 30; 312(7034): 843
KZ BMJ. 1996 Jan 20;312(7024):180-1. PMID: 8563545
PT comment; letter
VT
EDITOR - R W Lacey's letter makes no reference to any scientific paper on the epidemiology of bovine spongiform encephalopathy.[i] As a result it contains too many omissions, errors, and misconceptions to pass into the literature unchallenged.
Lacey's description of the cohort study to examine the risk of maternal transmission is incorrect. This study is comparing the incidence of bovine spongiform encephalopathy in offspring of dams that developed clinical signs of the disease and in offspring of dams that reached at least 6 years of age without developing clinical signs. Three hundred pairs of animals are involved, and the members of each pair were born in the same calving season and herd. The criteria for purchase, between July 1989 and February 1990, were that the animals had been weaned, females were virgin, males had been castrated, and documentation for the animals' provenance was available. Age at purchase ranged from 2 to 24 months; most of the animals were yearlings. The study population therefore comprises some animals born before the ban on ruminant feed containing recycled animal remains in July 1988. The animals are being kept until 7 years of age; the youngest will reach this age in November this year. The sample size of the study precludes interim analyses because of the inevitable loss of statistical power. The results of the study will be reported in due course; in the meantime not even Lacey can draw any conclusions.
Susceptibility to bovine spongiform encephalopathy is independent of age, but the risk of infection has undoubtedly declined as a result of the feed ban.[ii] Using a crude, inappropriate comparison of the age at which bovine spongiform encephalopathy has occurred in cattle born before and after the feed ban, Lacey claims that vertical and maternal transmission has occurred. He ignores a large case-control study that examined the risks of maternal and horizontal transmission in cattle born more than three and a half months after the feed ban, which provided substantial evidence against such means of transmission.[iii]
Furthermore, the low incidence of the disease m animals and the relatively constant incidences within herds[iv] are consistent with exposure to a low dose of infectious material and the occurrence of contamination in "packets."[v] In these packets the titre of the agent would vary, but there would not be any change in the infective dose with a change in the prevalence of infected cattle as Lacey infers. The mean age at clinical onset in cattle born after the feed ban has fallen as expected. Lacey will have to accept that there has been an incomplete observance of the feed ban.
JW WILESMITH Head of epidemiology department
Central Veterinary Laboratory, New Haw, Addlestone, Surrey KT15 0NE
i. Lacey RW. Creutzfeldt-Jakob disease and bovine spongiform encephalopathy. - BMJ 1996;312:180-1. (20 January)
ii. Hoinville LJ. Decline in the incidence of BSE in cattle born after the introduction of the "feed ban." Vet Rec 1994;134:274-5.
iii. Hoinville LJ, Wilesmith JW, Richards MS. An investigation of risk factors for cases of bovine spongiform encephalopathy born after the introduction of the "feed ban." Vet Rec 1995;136:312-8.
iv. Wilesmith JW. The epidemiology of bovine spongiform encephalopathy. Seminars in Virology 1991;2:239-45.
v. Kimberlin RH, Wilesmith JW. Bovine spongiform encephalopathy: epidemiology, low dose exposure and risks. Ann NY Acad Sci 1994;724:21O-20.
IN Der Autor moniert zu Recht Laceys Zitierfaulheit. Er selbst neigt aber zu unsinnigen Spekulationen. er behauptet durch den Vergleich von Nachkommen erkrankter Kühe mit denen nicht erkrankter Kühe gezeigt zu haben, dass es wahrscheinlich keine maternale Übertragung gebe. Dabei übersieht er, dass ein großer Teil der nicht erkrankten Kühe dennoch infiziert gewesen sein kann. Wenn alle Kühe das gleiche Futter gefressen haben, ist eine relativ gleichmäßige Infektion aller Mitglieder einer Herde wesentlich wahrscheinlicher als die die Vermutung des Autors, die Infektiosität liege packetweise vor. Ohne Erklärung hält er es für normal, dass das durchschnittliche Erkrankungsalter bei nach dem Verfütterungsverbot geborenen und dennoch an BSE gestorbenen Rindern geringer als früher üblich war.
ZR 5
MH Animal; Cattle; Cohort Studies; Disease Susceptibility; Disease Transmission, Horizontal; Disease Transmission, Vertical; Encephalopathy, Bovine Spongiform/*transmission; Female
SP englisch
PO England
OR Prion-Krankheiten 8